The influence of antiscalants on biofouling of RO membranes in seawater desalination

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 3 8 9 e3 3 9 8

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The influence of antiscalants on biofouling of RO membranes in seawater desalination Amer Sweity a, Yoram Oren a, Zeev Ronen b, Moshe Herzberg a,* a

Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Albert Katz International School for Desert Studies, Ben Gurion University of the Negev, Sede-Boqer Campus 84990, Israel b Department of Environmental Hydrology and Microbiology, Zuckerberg Institute for Water Research, Albert Katz International School for Desert Studies, Ben Gurion University of the Negev, Sede-Boqer Campus 84990, Israel

article info

abstract

Article history:

Antiscalants are surface active polyelectrolyte compounds commonly used in reverse

Received 4 January 2013

osmosis (RO) desalination processes to avoid membrane scaling. In spite of the significant

Received in revised form

roles of antiscalants in preventing membrane scaling, they are prone to enhance biofilm

16 March 2013

growth on RO membranes by either altering membrane surface properties or by serving as

Accepted 19 March 2013

nutritional source for microorganisms. In this study, the contribution of antiscalants to

Available online 29 March 2013

membrane biofouling in seawater desalination was investigated. The effects of two commonly used antiscalants, polyphosphonate- and polyacrylate-based, were tested. The

Keywords:

effects of RO membrane (DOW-Filmtec SW30 HRLE-400) exposure to antiscalants on its

Reverse osmosis

physico-chemical properties were studied, including the consequent effects on initial

Antiscalant

deposition and growth of the sessile microorganisms on the RO membrane surface. The

Biofouling

effects of antiscalants on membrane physico-chemical properties were investigated by

Fouling

filtration of seawater supplemented with the antiscalants through flat-sheet RO membrane

Scaling

and changes in surface zeta potential and hydrophobicity were delineated. Adsorption of

Biofilm

antiscalants to polyamide surfaces simulating RO membrane’s polyamide layer and their effects on the consequent bacterial adhesion was tested using a quartz crystal microbalance with dissipation monitoring technology (QCM-D) and direct fluorescent microscopy. A significant increase in biofilm formation rate on RO membranes surface was observed in the presence of both types of antiscalants. Polyacrylate-based antiscalant was shown to enhance initial cell attachment as observed with the QCM-D and a parallel plate flow cell, due to rendering the polyamide surface more hydrophobic. Polyphosphonate-based antiscalants also increased biofilm formation rate, most likely by serving as an additional source of phosphorous to the seawater microbial population. A thicker biofilm layer was formed on the RO membrane when the polyacrylate-based antiscalant was used. Following these results, a wise selection of antiscalants for scaling control should take into account their contribution to membrane biofouling propensity. ª 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Antiscalants (AS) are scale inhibitors polyelectrolytes (Smith, 1967) with reported optimal molecular weights in the range

of 1000e3500 g/mol, typically consisting of one component or a combination of polyphosphates, polyphosphonates, polyacrylates, and dendrimeric polymers (Farahbakhsh et al., 2004; Shih et al., 2006). AS have a superior effect avoiding

* Corresponding author. Tel.: þ972 8 6563520; fax: þ972 8 6563503. E-mail address: [email protected] (M. Herzberg). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.03.042

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precipitation of scale forming salts even at low dosage by preventing formation of crystal larger than a critical size (preventing nucleation) as well as by modifying the surface of larger crystals (Drak et al., 2000). In supersaturated salt solutions supplemented with AS, a significant delay in the induction time needed for precipitation is usually observed and the quality of the feed water is not affected (Fritzmann et al., 2007; Greenlee et al., 2009; Hasson et al., 1998, 2003; Gloede and Melin, 2006; Darton, 2000). In general, AS play crucial roles in maintaining efficient RO plant operations at the highest possible recovery by preventing the need to replace expensive membranes prematurely; eliminating or reducing the use of hazardous acids; producing less concentrate, reducing energy costs, as well as the downtime caused by frequent membrane cleaning (Greenlee et al., 2009; Gloede and Melin, 2006; Darton, 2000; Vrouwenvelder et al., 2000). Despite the significant roles of AS, they are reported to enhance membrane fouling, which is another setback for the optimum operation of any RO desalination process (Fritzmann et al., 2007; Greenlee et al., 2009; Vrouwenvelder et al., 2000, 2008; Baker and Dudley, 1998; Subramani and Hoek, 2008; Sadr Ghayeni et al., 1998; Kochkodan et al., 2008; Abd El Aleem et al., 1998; Fletcher, 1994; Ouazzani and Bentama, 2008; Chong et al., 2008). Accordingly, it is very important to use the minimum possible dosage of AS in order to avoid fouling enhancement at higher concentrations (AlShammiri et al., 2000). AS were shown to induce biofilm formation in RO systems by increasing the microbial growth potential up to 10 times of their normal growth rate (Vrouwenvelder et al., 2000, 2010). Chemicals used in water pretreatment in RO operations can promotes instability of colloids, particles and bacteria that can increase fouling rate (Winters, 1997). Biofilm formation is a progressive and developmental process initiated in bacterial deposition and irreversible adhesion to surfaces, followed by formation of micro-colonies encased in self-produced extracellular microbial matrix (also termed as extracellular polymeric substances e EPS), maturation, and finally, dispersion of the bacterial cells back to their planktonic stage (O’Toole et al., 2000). Deposition and attachment of the bacterial cells onto surfaces is a critical step in the overall process of biofilm formation, which is mediated by biological, physical, and chemical factors (Donlan, 2002). These factors includes substratum properties such as roughness, charge density and hydrophobicity (Diaz et al., 2010; Park et al., 2005); a “conditioning film” of macromolecules on the surface; hydrodynamics forces (Eshed et al., 2008; Purevdorj et al., 2002); solution chemistry such as ionic strength, pH, and the presence of multivalent cations (Chen et al., 2009; Chen and Walker, 2007; Rijnaarts et al., 1999); and bacterial cell surface properties such as hydrophobicity, expression of flagella and pili, lipopolysaccharides (LPS), and EPS. The reversible attachment of bacteria involves weak forces such as van der Waals, electrostatic and hydrophobic interactions between the bacterial cell and the substratum (Vanloosdrecht et al., 1987). Vrouwenvelder et al. showed how AS can enhance biofouling in RO membrane applications (Vrouwenvelder et al., 2000, 2010) by serving as phosphorous and carbon source of nutrients under limited nutritional conditions. While Vrouwenvelder et al. were focused on the nutritional effects of different AS on biofouling phenomena, in this

work, a broad perspective including both biological and physicochemical effects of AS on biofouling of RO membranes were investigated. One important AS effect includes the alteration of physico-chemical properties of either the membrane or bacterial surface due to AS adsorption, which consequently increases attachment of bacterial cells to the membranes’ surface.

2.

Materials and methods

2.1.

Model bacterial strain and media

Vibrio fischeri, a well-known marine bacterium used in this study, was obtained from Stritch School of Medicine in Loyola University, Chicago (http://www.meddean.luc.edu/lumen/ deptwebs/microbio/kv/kvmain.php). This bacterium, a green florescent protein ( gfp) chromosomally tagged mutant KV2682, was grown on LB agar medium supplemented with double the amount of NaCl (20 g/L), Trizma base (2-amino-2(hydroxymethyl)-1,3-propanediol) THAM 6.057 g/L, and 2.5 mg/L chloramphenicol. The strain was incubated and grown at 25  C and 250 rpm to final OD600 nm of 1. Then, the late exponential cells (after 5e6 h of incubation) were centrifuged (4  C, 2500 g, 20 min), washed three times, and resuspended to final OD600 nm of 0.1 with either filtered seawater or with seawater supplemented with 20 mg/L of each of the AS. Polyphosphonate- and polyacrylate-based AS analysis is shown the supplementary material (Table S1).

2.2.

Membrane preparation

A laboratory scale RO unit (Fig. 1) comprised a membrane cross-flow cell, high-pressure pump, feed water reservoir, chiller equipped with a temperature control system and PID pH controller for dosing CO2 gas and a data acquisition system. Permeate flow rate, conductivity, and pH were monitored in each experiment. A high flux RO flat-sheet membrane SW30 HRLE 400 (Dow-Filmtec, USA) was compacted with deionized water (DW) at a pressure of 60 bar before each experiment until the permeate flux attained a constant value, after 24 h. A pressure of 60 bar and a temperature of 25  C were kept constant during all the experiments. After the compaction stage completed, pretreated seawater sampled from Palmachim desalination plant (Palmachim, Israel) (after flocculation, coagulation, and sand and micronic filtration), with or without AS (20 mg/L) were filtered through the membrane for 24 h in the RO desalination lab unit. Then, the conditioned membranes were kept in 4  C, in similar seawater used for conditioning the membranes until further analysis of biofilm formation or surface properties characterization has been carried out. Usually, AS dosage in real RO installations varies between 2 and 8 mg/L. The reason for choosing 20 mg/L can be justified due to buildup of AS concentration at the tail end of the RO cascade due to recovery.

2.3.

Membrane surface properties

Prior to each biofilm formation experiment, after conditioning the membranes with seawater in the presence or absence of AS, membrane surface properties, i.e., surface zeta potential

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Fig. 1 e Cross-flow, flat-sheet, RO desalination unit used for conditioning RO membranes with AS during desalination of seawater.

and hydrophopicity, were measured. Membrane surface zeta potential was measured using a streaming potential analyzer (SurPass Elektrokinetic Analyzer) at 10 mM NaCl solution with or without pretreatment with AS. For each solution, measurements were done twice. During each measurement, each run of the electrolyte solution flow proceeded in two directions (right to left and then left to right). The zeta potential of the RO membranes was calculated from the streaming potentials using the HelmholtzeSmoluchowski equation with the Fairbrother and Mastin substitution (Benavente and Jonsson, 2000; Deshmukh and Childress, 2001). The hydrophobicity of the RO membrane was deduced from the contact angle analysis method, whereby it is determined by the captive bubble method (OCA, Data Physics). A droplet size of air water with diameter of 0.4e0.5 mm was introduced to the RO membrane surface after the antiscalant treatment stage in the RO unit. Duplicated experiments, with five different measurements of contact angle, were carried out for each of the treated membranes to obtain at least 10 measurements of contact angle for each set of conditions. In addition, X-ray photoelectron spectroscopy (XPS) was used to verify the effect of AS addition on the elemental composition of the polyamide surface membrane and properties after treating the membrane surface with seawater supplemented with or without AS. Elemental composition XPS analysis was deduced from deconvoluted spectra of carbon, nitrogen, oxygen, and phosphorous binding energies on membrane surface presented in the Supplementary material. Figures S1eS4, for pristine membrane as well as membranes treated with seawater, and seawater supplemented with either polyacrylate- or polyphosphonate-based AS, respectively.

2.4.

10 mM NaCl with and without antiscalant (20 mg/L). The relative hydrophobicity of the cells was determined by the “adhesion of microbial cells to hydrocarbon droplets” test (MATH test) with n-dodecane (Pembrey et al., 1999).

2.5. Monitoring bacterial attachment with quartz crystal microbalance with dissipation (QCM-D) Polyamide sensors mimicking RO membrane surfaces were used in E4 module QCM-D (Q-Sense, SWEDEN) for analyzing the effect of AS on bacterial deposition and attachment. All QCM-D experiments were performed at flow-through conditions using a digital peristaltic pump (IsmaTec Peristaltic Pump, IDEX) operating in a pushing mode. The flow rate of the working solution in the QCM-D flow cell was 150 mL/min. The following solutions were injected sequentially to the QCM-D system: (i) double distilled water baseline for 20 min; (ii) 0.2 mm filtered seawater for 20 min; (iii) seawater supplemented with 20 mg/L AS; (iv) bacterial suspension in 0.2 mm filtered seawater with or without AS for 20 min; and (v) finally, these steps were repeated in a reverse order. When no AS was supplemented, step (ii) was injected for 40 min. The adsorption kinetic curves were made by Q-Tools software (Q-SENSE, Sweden). This software adjusts the incoming and outgoing electrical currents, regulates the amplitude of the oscillation and controls the temperature according to the temperature set. The variations of frequency shift (Df, Hz) and dissipation factor (DD) were measured for five overtones (n ¼ 3, 5, 7, 9 and 11) and the 7th overtone is presented.

2.6. Bacterial deposition experiments using parallel plate flow cell

Bacterial physico-chemical properties

The electrophoretic mobility of the bacterial cells was measured by zeta potential analyzer (ZetaPlus 1994, Brookhaven instruments Co., Holtsville, NY). Cells were washed in

After treating both the RO membranes and the bacterial cells with AS, three sets of duplicated deposition experiments were carried out to characterize the effect of AS treatment on the deposition and attachment of the bacterial cells on the RO

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membrane surface. AS concentration used to treat bacterial cells and RO membranes was 20 mg/L. In each set of experiments, characterization of the initial bacterial deposition to the RO membrane surface was done by placing a piece of each of the treated RO membranes in a parallel flow cell (FC81, Biosurface Technologies, Bozeman, Montana). Then, the treated bacterial suspension was injected into the flow cell (140.6 mm long  12.7 mm wide  0.20 mm deep) at a velocity of 8.5 cm/s. The accumulated bacteria on the membrane surface were monitored with a fluorescent microscope (Zeiss AXIO Imager) with magnification of 10X and every image was taken at fixed spot of the membrane. The monitored spots on the membrane coupon were visualized every 20 min and acquired with a CCD camera. At least 5 pictures were taken every 20 min, and the fluorescent signals of the bacteria were manually counted on a rectangular viewing area of 861  650 mm, which was recorded by the CCD camera at a magnification of 10. Before snapping the pictures, the same background solution as in the experiment (without bacteria) was injected into the flow cell for 20 s for washing the suspended bacteria in order to visualize only the attached cells. This way, each set of experiment was carried out for three hours and the gradual increase in the number of deposited bacteria was recorded and the number of deposited bacteria per cm2 of the observed RO membrane surface was calculated. As mentioned, at each time point, 5 different spots on the membrane were visualized and the number of the deposited cells was averaged, calculated per unit of area and normalized to the initial cell concentration in each experiment. The deposition coefficient was calculated as the number of deposited cells per min per cm2 and normalized to the injected bacterial cell concentration (Vanoyan et al., 2010).

2.7. Biofilm growth experiments on RO membranes in a parallel plate flow cell The conditioned RO membranes from the RO lab desalination unit (Fig. 1) were used for biofilm growth experiments, which were conducted in a parallel plate flow cell (FC81, Biosurface Technologies, Bozeman, Montana). Seawater with or without AS (100 mg/L) were injected into the flow cell, which was occupied with SW30 membrane. It should be mentioned that enhanced biofilm growth experiments were conducted with a relatively high concentration of AS in order to compare between the AS being used and to achieve biofilm growth within a reasonable experimental period. The flow cell dimensions were 140.6 mm long  12.7 mm wide  0.20 mm deep and a flow velocity of 8.5 cm/s was kept constant for ten days period of biofilm growth. The AS concentration (100 mg/L) in the inoculum was relatively high, in order to boost the bacterial growth during the relatively short experimental period. The microbial inoculum used for this experiment, was bacterial culture cultivated for four months prepared as following: sterile flasks (250 mL) with 150 mL of Palmachim desalination plant treated seawater supplemented with AS were incubated for 4 months at 30  C and 250 rpm. A weekly replacement of 140 mL of seawater with or without antiscalant was carried throughout the 4 months period in order to create a selective pressure for the growth of the antiscalant-degrading bacteria. Each of the cultures was used as inoculum for the biofilm

growth experiments according to the antiscalant being analyzed. At the end of each experiment, the membrane was collected for confocal laser scanning microscope (CLSM) analysis. Prior to the CLSM, pieces of the membranes (5 mm  5 mm) from the flow cell were cut from the middle of the membrane coupon and fixed immediately for all runs, without drying the membranes. The fixations of the fouled membranes were done by adding 0.05 M sodium cacodylate buffer supplemented with 2% glutaraldehyde for 1 h. Then the fouled membranes were stained with concanavalin A (ConA) conjugated to Alexa Fluor 633 (Invitrogen, Israel) and with propidium iodide (PI) for probing EPS and cells, respectively. Microscopic observation and image acquisition were performed using Zeiss-Meta 510, a CLSM equipped with Zeiss dry objective LCI Plan-NeoFluar (25X magnification and numerical aperture of 0.8). CLSM images were generated using the Zeiss LSM Image Browser. Gray scale images were analyzed, and the specific biovolume (mm3/mm2) was determined with IMARIS v7.5 software (IMARIS Bitplane, Zurich, Switzerland). Table 1 summarizes the types of bacterial cultures used in the different deposition and biofouling experiments.

3.

Results and discussion

3.1. Effect of antiscalants on membrane surface properties The effect of AS on the surface physico-chemical properties of the Filmtec SW30 RO membranes was determined by filtration of Palmachim desalination plant feed seawater with or without AS through the membranes for 24 h (Fig. 1).

3.1.1.

Zeta potential effect

For all cases, at 10 mM NaCl, membranes’ surface zeta potential was negative for pH values above 3.8 (Fig. 2A). At lower pH, both AS increased membrane positive charge: a neutral zeta potential was observed for polyacrylate-based AS at pH below w3.5 and slightly higher positive zeta potential was observed for the membrane treated with polyphosphonatebased AS. Clearly, when the membrane was not exposed to any of the AS, the membrane was more negatively charged for the entire pH range.

Table 1 e Types of bacterial cultures used in this research study. Experiment Bacterial physico-chemical properties Bacterial deposition and attachment using QCM-D Bacterial deposition and attachment experiments using parallel plate flow cell Biofilm growth experiments on RO membranes in a parallel plate flow cell

Type of culture V. fischeri pure culture V. fischeri pure culture V. fischeri pure culture

Natural microbial consortium isolated from a fouled RO membrane

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Hydrophopicity effect

Using captive bubble contact angle method (Zhang and Hallstro¨m, 1990) enabled us to deduce the effect of polyacrylate-based and polyphosphonate-based AS under most realistic conditions: upon exposing to seawater in the absence of AS, the membrane surface was relatively hydrophilic (Fig. 2B) with a contact angle of 32  1.5 . Interestingly, when exposed to seawater in the presence of AS, a significant increase in hydrophobicity of the RO membrane surface was observed with contact angle measurements of 54.3  5.4 , and 42  4.3 for the membranes treated with polyphosphonatebased and polyacrylate-based AS, respectively (Fig. 2B). While polyacrylates are linear with amphiphilic structure, the positive shift in zeta potential and the increased hydrophobicity of the membrane exposed to polyacrylate-based AS, implies that carboxylic moieties on this AS interact with the RO membrane, probably chemically cross-bridged with divalent cations as Caþ2 with the hydrophobic tail of this AS exposed to aqueous surrounding environment. It should be mentioned that zeta potential analysis of the membranes was conducted in 10 mM NaCl after treating the membrane with seawater with or without AS. The results imply insignificant possible washout of the AS from the membranes during the streaming potential analysis. Since polyphosphonate and aminophosphonates, which mainly comprise the polyphosphonate-based AS, are smaller in their size and hydrophilic than polyacrylates, almost no effect on membrane hydrophobicity was observed. However, interactions of both of carboxylic and phosphate moieties of the polyphosphonate and the membrane surface interaction will be facilitated by Caþ2 cations (Guo and Severtson, 2004; Petit-Agnely et al., 2000; Skwarczynski et al., 2010; Tsiourvas et al., 1997).

3.1.3.

XPS analysis

The SW30 HRLE 400 polyamide membranes treated with seawater supplemented with AS were characterized using XPS to monitor the presence of AS fingerprinting on the membrane surface as shown in Table 2. XPS is a sensitive surface technique providing analysis of surface chemical groups in a depth range of about 5e10 nm (Ferjani et al., 2000; Williard, 2007). From the deconvoluted binding energy

Zeta potential, mV

10

Blank CA PP

0 -10 -20 -30 -40

2

4

6

pH

8

10

spectra, five different types of carbon bonds, two different types of nitrogen bonds, three types of oxygen bonds, and two types of phosphorous bonds were revealed. The different peaks of the binding energies (BE), the related bonds found in previous studies (Ferjani et al., 2000; Williard, 2007; Boussu et al., 2007; Liu et al., 2006; Tang et al., 2007, 2009), and our suggested origin of the peaks (membrane or both membrane and AS) are summarized in Table 2. As it was impossible to distinguish between bonds originated from AS and the membrane due to similarity of the bonds being detected, evidences for the AS adsorption to the membrane were absence of the aromatic CeC/CeH peak of 285 eV and the CH2eNCO peak of 288 eV, only for the membrane treated with polyphosphonate-based AS (Table 2). In addition, for the membrane treated with carboxylic acid-based AS, the nitrogen peak that is attributed to amine group (with binding energy BE between 400.76 and 400.85) was not detectable, probably due to interaction of carboxylic acids with a surface amine group. For the oxygen peaks, as presence of the three main types of oxygen was detected on all membranes, and therefore oxygen bonds were not useful to determine the presence of adsorbed antiscalant. Still, the absence of O 1s peak at BE around 529, rules out the presence of any metal oxides on the membrane surface. Regarding the phosphorous bonds, the virgin membrane had the least amount of phosphorous: the atomic percentage of phosphorous on the membranes were 0.19%, 0.3%, 0.26% and 0.54% for the virgin membrane, membrane treated with seawater, membrane treated with seawater and polyacrylate-based AS and membrane treated with seawater and polyphosphonates-based AS, respectively (Table 2). As expected, the membrane treated with polyphosphonate-based AS showed the highest percentage of phosphorous.

3.2. Effect of antiscalants on bacterial physico-chemical properties 3.2.1.

Hydrophopicity effect

The effect of AS on the relative hydrophobicity of the V. fischeri KV2682 bacterial cells was analyzed. Interestingly, when treated with 20 mg/L polyphosphonate-based AS (PP), these bacteria showed lower hydrophobicity compared to the

Contact angle ,Degrees

3.1.2.

60

40

20 Blank

CA

PP

Membranes condtioned

Fig. 2 e The effect of antiscalant treatment (20mg/L) on membrane surface charge and hydrophobicity: polyphosphonatebased (PP) and polyacrylate-based (CA) AS were tested. (A) Filmtech-SW30 RO membranes surface zeta potential plotted as a function of the pH in a background solution of 10 mM NaCl; (B) Captive air bubble contact angle on the surface of FilmtechSW30 RO membrane.

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Table 2 e Elemental compositions were computed based on C (1s) N (1s), O (1s), and peaks, which are centered around 532, 399, and 284 eV, respectively. BE refers to the peak binding energy and % At refers to the elemental percentage on the surface of the membrane for 4 different membranes. Samples are pristine membrane, membrane treated with seawater only, membrane treated with seawater supplemented with carboxylic acid-based AS and a membrane treated with seawater supplemented with polyphosphonate-based AS. Suggested bond Peak BE

At %.

Suggested origin

Peak BE

At %.

284.65 286.31 288.14 285.41

52.51 22.12 10.07 10.39

Membrane Membrane Membrane Membrane

284.71 286.45 288.4 285.49

49.39 25.55 11.26 9.94

287.33

4.9

Membrane

287.6

399.79 400.85

67.08 32.92

Membrane Membrane

O1S scan e no metal oxides (absence of peaks at BE ¼ 529) COO or OH 532.59 42.3 Membrane 533.43 31.8 Moisture H 2O O]CeN 531.53 25.9 Membrane P2p scan CeP or PO 133.57 0.19 Membrane

Seawater þ CA

Seawater w/o AS Suggested origin

Seawater þ PP

Peak BE

At %.

Suggested origin

Peak BE

At %.

Suggested origin

Membrane Membrane Membrane Membrane

284.62 286.38 288.43 285.43

49.48 22.07 11.78 11.57

Membrane or AS Membrane Membrane or AS Membrane

284.59 46.32 Membrane or AS 286.13 39.82 Membrane Non-detectable: covered by layer of AS Non-detectable: covered by layer of AS

3.86

Membrane

287.44

5.09

Membrane

287.86

13.86

Membrane (shifted)

400.04 401.49

92.69 7.31

Membrane Membrane (shifted) or ammonium

400.04 100 Non-detectable: interactions of COO with amine groups

399.74 400.76

65 35

Membrane Membrane

532.54 533.69 531.41

58.78 22.67 18.55

Membrane Membrane Membrane

532.38 533.45 531.42

41.5 30.52 27.98

Membrane or AS Moisture Membrane or AS

532.47 533.76 531.17

62.51 17.33 20.16

Membrane or AS Moisture Membrane or AS

133.74

0.30

Membrane and seawater

133.62

0.26

Membrane and seawater

132.74

0.54

Membrane and seawater þ AS

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 3 8 9 e3 3 9 8

C1S scan Aliphatic CeC/CeH CeO/CeN COO orCH2eNCO Aromatic CeC/ CeH stretch O]CeN N1S scan O]CeN Amine

Pristine membrane

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 3 8 9 e3 3 9 8

polyacrylate-based AS as well as to the case without AS (Fig. 3A). The values for the percent of partitioning between the aqueous and the organic phase were 17%  0.62, in the absence of the AS; 14%  0.42 in the presence of polyphosphonate-based AS; and 28%  0.54, in the presence of polyacrylate-based AS (Fig. 3A).

3.2.2.

Zeta potential effect

The electrophoretic mobility of the V. fischeri KV2682 cells treated with different AS is shown in Fig. 3B. The results show no significant effect on the zeta potential of this strain by the presence of both AS, with zeta potential values for all cases in the range between 35 and 38 mV. Similar to the membrane surface, changes in bacterial cell hydrophobicity in the presence of polyacrylate- and polyphosphonate-based AS are also affected by the chemical nature of these compounds and the effects are similar. Since bacterial cells were washed with 10 mM NaCl and in the absence of Caþ2, prior to their zeta potential measurements with the different AS, no change in zeta potential was observed, in contrast to the membrane surface, significantly affected by prior conditioning filtration step of seawater in the RO lab unit.

3.3.

Antiscalants effects on bacterial deposition

In order to study bacterial deposition and the related physical and chemical interactions, model bacterium with defined properties must be used. Clearly, a pure strain does not represent the actual behavior of natural microbial consortium in the environment but it could be the guidance for such phenomena. The selected model strain, V. fischeri KV2682, is a representative marine bacterium and its expression of a green florescent protein ( gfp) allows precise microscopic tracking. Recently, Naidu et al. (2013) used V. fischeri to investigate the role of microbial activity in biofilter used as a pretreatment stage for seawater desalination. Here, the effects of AS on bacterial attachment were conducted on a polyamide coated sensor in a QCM-D flow cell. First, a baseline is achieved with double distilled water (DDW) for 20 min to assure that the polyamide coated sensors is absolutely clean. Then, filtered seawaters were injected as a background solution, followed by injection of seawater with or without antiscalants used to condition the polyamide surface. After conditioning the surface with seawater in the presence or absence of antiscalant, bacterial suspension was injected under similar aquatic conditions. Frequency shift of the

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sensor is shown later in this study, to be positively related to the amount of attached/detached bacteria to/from the sensor as confirmed by direct fluorescent microscopy. The results in Fig. 4 show the AS effects on the adherence of the bacterium V. fischeri KV2682. In the presence of the polyacrylate-based AS, the bacteria cells attached to the sensor at higher extent compared to their attachment in the presence of the polyphosphonate-based AS, since a higher decrease of the frequency was observed for the case using polyacrylate-based AS. These results can be related to the combination of higher hydrophobicities of the bacterial cells and polyamide surface treated with polyacrylate-based AS as well as to the more positive zeta potential of the AS treated membrane (Figs. 2 and 3). Yet, we do not have an explanation for the lower adhesion of bacteria after treatment with polyphosphonate-based AS, where in fact, the bacteria were excluded from the polyphosphonate treated surface, while attracted to the polyacrylate treated one. Possible reason may be the lower hydrophobicity of the bacteria treated with polyphosphonatebased AS as shown in Fig. 3A. Careful interpretation of the frequency shift acquired during bacterial deposition on QCMD sensors should be taken into account, as the frequency can be influenced by cell surface morphology with some cases, where frequency shift is not positively related to the increase of bacterial adhesion (Marcus et al., 2012). Therefore a positive relation between deposition of bacterial cells and frequency shift was established: similar trend for the antiscalant effect on the bacterial cells attached to the RO membrane surfaces was observed using direct fluorescent microscopy with this bacterium as shown in the inset graph of Fig. 4.

3.4.

The effect of antiscalants on biofilm formation

Biofilm growth experiments conducted using parallel plate flow cell were conducted and the effect of the polyacrylateand polyphosphonate-based AS on biofilm formation by natural microbial consortium was delineated. As already mentioned, prior to the injection of the seawater, with or without the AS, inoculums from the incubated seawater were supplemented to the flow cell in order to boost the biofilm growth process. CLSM analysis of membrane samples taken from the parallel plate flow cell shows that addition of antiscalant promoted biofilm growth on the surface of the RO membrane (Fig. 5). Specific biovolume analysis of the biofilms using IMARIS software (Bitplane, Switzerland) on the membranes showed that polyacrylate treated membrane had more

Fig. 3 e The effect of antiscalant treatment (20 mg/L) of polyphosphonate (PP) and polyacrylate (CA) based AS on the relative hydrophobicity (A) and the zeta potential (B) of Vibrio fischeri KV2682 bacterial strain in 10 mM NaCl at ambient pH of 6.2.

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DDW

SW

SW+AS

-6

DDW

SW

3.0x10

0

Frequency, Hz X10

Bacteria SW+AS 2.5x10 2.0x10

-5

1.5x10 1.0x10

-10

5.0x10 0.0

-15

SW

CA

PP

Treatment

-20

PP

-25

CA Blank

-30 0

20

40

60 80 Time, Minutes

100

120

140

Fig. 4 e The effect of antiscalant addition on the attachment of bacterial cells to polyamide coated QCM-D sensor as measured by change in frequency of oscillation of the 7th overtone. Inset graph describes the deposition coefficient for GFP tagged Vibrio fischeri on SW30 RO membrane surfaces during cross-flow of 2 mL/min in a parallel plate flow cell (FC81 flow cell, Biosurface Technologies, Bozeman, Montana).

biomass in comparison to the case of polyphosphonate treated membranes and the membrane without antiscalant (Fig. 5). While both polyacrylate- and polyphosphonate-based AS increased biofilm formation, the increased bacterial attachment analyzed for polyacrylate-based AS induced more effectively biofilm formation process. Polyphosphonate, on the other hand, had an opposite effect on bacterial attachment that could not explain the moderate elevation in biofilm formation as presented for this case in Fig. 5. The moderate increase in biofilm formation in the presence of polyphosphonates could be attributed to the supplement of a limiting nutritional element for microbial growth under

phosphorous limiting growth conditions in seawater (undetectable level of total P). Note that in real RO installations, a lower concentration of AS will be used, but still may reach close to the levels used in this study, due to seawater desalination recovery of around 50%. Likely, the lower AS concentration being used, but still efficient in avoiding membrane scaling, the lower biofouling side effects will rise. In should be mentioned that more study need to be done in order to understand the contribution of each antiscalant to biofilm growth. This can be achieved by measuring continuously other biofilm growth-related parameters, including the presence of adenosine triphosphate (ATP) in the biofilms, and

Fig. 5 e Left panel represents three-dimensional reconstructed images acquire from CLSM using Imaris Bitplane software (the red colour represents biomass and the green colour represents EPS) of the fouled SW30 RO membrane surface after injecting seawater without AS (A); seawater supplemented with polyacrylate AS (B); and seawater supplemented with polyphosphonate AS (C). The resolution of the perspective images is 450 3 450 mm. Right panel (D) represents the amount of biomass per unit area of attached bacterial cells and adsorbed EPS in the biofilm layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 3 8 9 e3 3 9 8

phosphorous and total organic carbon (TOC) concentration in the feed and effluent water of the parallel plate flow cell.

4.

Concluding remarks

While scale formation in RO desalination systems can be successfully prevented by AS, they can enhance biofilm formation on RO membranes during seawater desalination in several ways, at different stages of biofilm growth. Polyacrylate-based AS were shown to enhance biofilm formation, most likely by altering the physico-chemical properties of the RO membranes such as hydrophobicity and surface charge, which in turn promote the initial deposition and attachment of bacterial cells. Polyphosphonate-based AS were shown to contribute membrane biofouling probably by acting as a phosphorous source of nutrients under the phosphorous limitation conditions prevailing in seawater. Comparing these two effects shows that the physico-chemical effects of AS can severely increase RO membrane biofouling. Based on the findings of this work, it is strongly recommended that AS should be screened for their biofouling contribution and related enhancing steps such as particulate and microbial attachment, as well as nutrient source, prior to their application.

Acknowledgements This research study was funded by the Israel Governmental Authority for Water and Sewage and by Albert Katz International School for Desert Studies (AKIS) in Ben-Gurion University of the Negev, which partially supported Amer Sweity’s PhD fellowship.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.03.042.

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